1. Field of the Invention
The present invention relates to an electro-photographic image forming apparatus, such as a printer, a copier, etc., and in particular, to an image forming apparatus capable of synchronizing a sheet in a transfer process without temporary stopping the sheet during sheet conveyance.
2. Discussion of the Background Art
An optical scanning type image forming apparatus such as an electro-photographic image forming apparatus is widely used in a LBP (Laser Beam Printer), a facsimile, or the like, due to its high speed and high-resolution performances. Such an electro-photographic image forming apparatus generally includes a rotational photoconductive drum and various sections, such as a charger for charging the photosensitive surface of the photoconductive drum, an exposure section for exposing the photoconductive surface with charge and form a latent image, a developing section for supplying toner and making the latent image into a toner image, and a transfer section for transferring the toner image onto a sheet, around the photoconductive drum.
In the image forming apparatus, a polygon mirror constantly rotates and scans a laser beam emitted from a semiconductor laser to execute exposure on the surface of the photoconductive member in a main scanning direction. A polygon motor for driving the polygon mirror is conventionally in the standby state, and starts rotating as image formation starts. A sheet is conveyed and stopped at a prescribed position right before a transfer position. Then, sheet feeding is restarted by a registration clutch mechanism, for example, after a prescribed time has elapsed. That is, the sheet restarts for the purpose of waiting steady state rotation and synchronization with the image formation. However, since reach of the steady state rotation is not always detected conventionally, the prescribed time tends to be unnecessarily longer, sometime, thereby resulting in slow printing of the first sheet.
Then, Japanese Patent Application Laid Open No. 5-2298 attempts to employ a detection device for detecting if a polygon motor reaches steady state rotation and for restarting sheet feeding when the steady state rotation is detected in a registration clutch using image forming apparatus. Further, many low price machines do not generally include a registration clutch for the purpose of reducing cost.
In such a conventional image forming apparatus, the slow printing out can be avoided indeed, but is costly due to extra employment of the registration clutch. Further, the low price machine generally causes deviation of an image on a transfer sheet due to omission of synchronization between the image and the sheet at a transfer position.